No. 104.] 179 



were peninsulas or promontories of older gneisses in the later schists 

 and associated crystalline rocks. The line may be described as located 

 south-west of Croton falls ; near Somers village at the southern 

 end of the Eound hill range ; then, bending to tiie north-west and 

 near the so-called " Lovell street," it strikes the Putnam county line 

 near the Plum brook depression and the Mahopac Branch railway; 

 then bending southward and on a west-south-west course it re-enters 

 Westchester county at Jefferson valley; thence it is traced westward 

 to Shrub Oak along the southern foot of a high and rocky ridge of 

 thick-bedded, grey gneiss, which runs northward along on the south- 

 east of the Peekskill hollow. It should be stated here that there are 

 outcrops of true gneiss in the high ground south of Shrub Oak and 

 Jefferson valley, and as far south as Yorktown, which may belong to 

 the Highlands formation, and if so included, the location of this line 

 must be somewhere near the west branch of the Muscoot river and 

 south of Yorktown village. As no connection could be traced between 

 these more southern outcrops and the main body of Arclipoan to the 

 north, it is believed that they are parts of an isolated area in this 

 (Y^'orktown) township. The thin-bedded, mica schists which have a 

 prevailing east and Avest strike, and which make the more gently roll- 

 ing country in the central part of Somers and Yorktown townships, 

 are here considered as a distinct formation on the south of the Archa'au 

 gneisses of the rocky and mountainous Highlands. From Shrub Oak 

 the direction of this boundary is west a short distance ; thence north- 

 west, again into Putnam county, nearly to Adams' Corner; thence 

 south-west, down the valley of the Peekskill Hollow creek, north of 

 Oregon, near which place it returns into Westchester county. From 

 Oregon the line is traced north of Gallows hill to Sprout brook and 

 thence to Annsville, and the Peekskill Cove to the Hudson river. 

 North of Oregon the rock outcrops on the south-east, adjacent to the 

 gneisses is a black crenulitic slaty rock which resembles closely the 

 rock near Annsville, and that in the West Shore railroad cuts north 

 o Tomkins Cove station. At the latter place the rock is graphitic 

 and a hydro-mica schist rather than a true argyllyte. 



The limestone belt in the Sprout Brook or Canopus Hollow is not 

 here defined in its location, inasmuch as it is placed with the gneisses, 

 as an Archaean rock ; its semi-crystalline character and its foreign 

 minerals, and its want of resemblance to the Peekskill Hollow lime- 

 stone appear to indicate its close relation, to the gneissic rocks adjacent 

 to it. The general direction of strike and dip in these slaty outcrops, 

 bordering the Highlands from Peekskill cove to Oregon, is like that 

 of the Archaean gneisses, the prevailing dip being at a steep angle 

 toward the south-east. The absence of any localities where contacts 

 are to be seen makes the relative position uncertain. The observations 

 of this reconnaissance do not prove unconformability. A more thor- 

 ough survey of all the outcrops is needed to discover the true relations 

 so far as structure is concerned. 



On the eastern side of the Highlands the Arch;pan border has the 

 micaceous, schistose rocks and the quartzites resting upon it; and two 

 localities near Towner's station in the town of Patterson, Putnam 

 county, may be here noted, on account of their closely contiguous 

 outcrops of the bottom, granitoid gneisses and the upper schists and 

 quartzose rocks. The first one to be mentioned is at the south end 



